1. Field of the Invention
The present invention relates to the field of fetal genetic screening and to the field of quantitative nucleic acid analysis.
2. Related Art
It is now recognized that fetal DNA sheds from the placenta and mixes with the mother's blood at fairly high levels—between 3% and 6% of DNA in the mother's blood is from the fetus. This observation has been used in conjunction with PCR assays for a variety of fetal genetic screens—gender, Rh, and thalassemia. However, the technique remains limited for two primary reasons: first, the PCR assays trade off sensitivity for specificity, making it difficult to identify particular mutations, and second, the most common genetic disorder, Down Syndrome, is a chromosomal trisomy and therefore cannot be detected by conventional PCR in a mixed sample.
It has now been found that these problems can be solved by quantitative examination of large numbers of chromosome samples through the use of highly scalable techniques. This approach is termed here “digital analysis,” and involves the separation of the extracted genomic material into discrete units so that the detection of a target sequence (e.g., chromosome 21) may be simply quantified as binary (0, 1) or simple multiples, 2, 3, etc. The primary example of a technique that can be used to yield such “digital” results is “digital PCR,” which allows efficient amplification from single molecules, followed by subsequent quantitative analysis. Digital PCR, as the term is used here, refers to a quantitative, limited dilution of a nucleic acid sample, such as into multiwell plates, then the amplification of a nucleic acid molecule in a well, which due to the dilution, should be either 0 or 1 molecule. Digital PCR using multiwell plates has been used previously to detect rare mutations by either serial analysis of single molecule (i.e., clonal) amplicons (Vogelstein B, Kinzler K W. Proc Natl Acad Sci U S A., 1999 Aug. 3; 96 (16): 9236-41) or by enhancing the sensitivity of differential amplification (www(dot)fluidigm.com/didIFC.htm). Described below is an invention whereby digital PCR can be applied to noninvasive fetal diagnostics in order to detect fetal mutations with specificity and sensitivity beyond what is possible with conventional PCR analysis.
Furthermore, as also described in connection with the invention described below, digital PCR can be used to detect aneuploidy, such as the trisomy that causes Down Syndrome. Since aneuploidies do not present a mutational change in sequence, and are merely a change in the number of chromosomes, it has not been possible to detect them in a fetus without resorting to invasive techniques such as amniocentesis or chorionic villi sampling (Science 309, 2 Sep. 2005 pp. 1476-8).
Another form of digital PCR has been described as emulsion PCR, which has been used to prepare small beads with clonally amplified DNA—in essence, each bead contains one amplicon of digital PCR. (Dressman et al, Proc Natl Acad Sci U S A. 100, 8817 (Jul. 22, 2003)).
Another form of Digital PCR can be carried out using microfluidics. In this embodiment, described below, DNA is diluted and separated into small, discrete samples for forming reaction samples by a series of channels and valves.
An example of a suitable method for single molecule analysis that may be adapted to the present methods is given in Braslaysky et al., “Sequence information can be obtained from single DNA molecules, Proc. Nat. Acad. Sci. 100(7): 3960-3964 (2003), which uses sequential incorporation of labeled nucleotides onto an immobilized single stranded DNA template and monitoring by fluorescent microscopy.
Another aspect of the relevant art involves sample preparation in order to carry out the present processes. That is, the fetal DNA may be enriched relative to maternal DNA. Chan, et al., “Size Distribution of Maternal and Fetal DNA in Maternal Plasma,” Clin. Chem. 50(1): 88-92 (2004) reports that plasma DNA molecules are mainly short DNA fragments. The DNA fragments in the plasma of pregnant women are significantly longer than DNA fragments from non-pregnant women, and longer than fetal DNA.
Related Publications and Patents
Vogelstein et al., “Digital Amplification,” U.S. Pat. No. 6,440,706, issued Aug. 27, 2002, discloses the identification of pre-defined mutations expected to be present in a minor fraction of a cell population.
Lo, “Fetal DNA in Maternal Plasma: Biology and Diagnostic Applications,” Clin. Chem. 46:1903-1906 (2000) discloses the demonstration of fetal DNA in maternal plasma. The authors found a mean fractional level of 3.4% fetal DNA in maternal DNA in plasma during early pregnancy. The authors report detection of the RhD gene and microsatellite polymorphisms in the plasma of pregnant women.
Li et al., “Detection of Paternally Inherited Fetal Point Mutations for β-Thalassemia Using Size Fractionated Cell-Free DNA in Maternal Plasma,” J. Amer. Med. Assoc. 293:843-849 (Feb. 16, 2005) discloses that the analysis of cell-free fetal DNA in maternal plasma has proven to be remarkably reliable for the assessment of fetal loci absent from the maternal genome, such as Y-chromosome specific sequences or the RhD gene in pregnant women who are Rh-negative. The authors report on the extraction and size fractionation of maternal plasma DNA using agarose gel electrophoresis. Then, peptide-nucleic acids (PNA) were used to bind specifically to a maternal allele to suppress PCR amplification of the of the wild type maternal allele, thereby enriching for the presence of paternally inherited mutant sequences. Four distinct point mutations in the β-globin gene were examined. It was found that the PNA step was necessary for the detection of mutant alleles using allele specific PCR.
Lo et al., “Quantitative Analysis of Fetal DNA in Maternal Plasma and Serum: Implications for Noninvasive Prenatal Diagnosis,” Am. J. Hum. Genet. 62:768-775 (1998) discloses a real-time quantitative PCR assay to measure the concentration of fetal DNA in maternal plasma and serum. The authors found a mean of 25.4 genome equivalents/ml of fetal DNA in early pregnancy. This corresponds to about 3.4% of total DNA in early pregnancy.
Chan et al., “Size Distribution of Maternal and Fetal DNA in Maternal Plasma,” Clin. Chem. 50:89-92 (January 2004) investigated the size distribution of plasma DNA in non-pregnant women and pregnant women, using a panel of quantitative PCR assays with different amplicon sizes targeting the leptin gene. They found that the DNA fragments in the plasma of pregnant women are significantly longer than those in the plasma of non-pregnant women, and the maternal-derived DNA molecules are longer than the fetal-derived ones.
Tufan et al., “Analysis of Cell-Free Fetal DNA from Maternal Plasma and Serum Using a Conventional Multiplex PCR: Factors Influencing Success,” Turk. J. Med. Sci. 35: 85-92 (2005) compared the success rates of two different DNA extraction techniques, the heat based direct method and the QIAMP DNA blood mini kit method. The crucial role of PCR optimization was also reported. The authors used the DYS14 marker for the Y chromosome and the GAPH gene for a control. The QIAMP mini kit was found to give the best results in sex determination analysis using multiplex PCR and ethidium bromide staining on gels.
Hromadnikova et al., “Quantitative analysis of DNA levels in maternal plasma in normal and Down Syndrome pregnancies,” BMC Pregnancy and Childbirth 2(4): 1-5 (2002), investigated total DNA levels in maternal plasma and found no difference in fetal DNA levels between the patients carrying Down Syndrome fetuses and the controls. Real time quantitative PCR analysis was performed using primers to the β-globin gene and the SRY locus.
Grundevikk and Rosen, “Molecular Diagnosis of Aneuploidies,” published on line at www(dot)molbiotech.chalmers.se/research/mk/mbtk/Molecular%20diagnostics%20of%20aneuploidies%20-%20rapport.pdf, suggests that non-invasive methods for detection of aneuploidies (such as Down Syndrome, Edwards Syndrome or extra sex chromosomes) may be carried out on fetal nucleated cells isolated from maternal blood. In their review, the authors also describe quantitative fluorescence polymerase chain reaction (QF-PCR), based on amplification of short tandem repeats specific for the chromosome to be tested. They describe tests where DNA was amplified from amniotic or chorionic villus samples. The authors suggest that the STR markers will give PCR products of different size, and these size differences may be studied by analyzing peak sizes in electrophoresis. It is also proposed that quantitative real time PCR may be used to diagnose Down Syndrome by comparing the amount of a gene located on chromosome 12 to the amount of a gene located on another autosomal chromosome. If the ratio of these two genes is 1:1, the fetus is normal, but if the ratio of these genes is 3:2, it indicates Down Syndrome. The authors propose the use of Down Syndrome marker DSCR3. They also suggest that the housekeeping gene GAPDH on chromosome 12 can be used as a reference.
Poon et al., “Differential DNA Methylation between Fetus and Mother as a Strategy for Detecting Fetal DNA in Maternal Plasma,” Clin. Chem. 48(1): 35-41 discloses the detection of genes or mutations in a fetus where the same mutation or condition is also present in maternal DNA. That is, the use of fetal DNA in maternal plasma is limited due to the low amount of fetal DNA compared to maternal DNA. The authors overcame this limitation by detecting the IGF2-H19 locus, which is maintained in a methylated DNA status in the paternal allele and is unmethylated in the maternal allele. The authors used a bisulfite modification kit whereby unmethylated cytosine residues were converted to uracil. The sequence difference between methylated and unmethylated DNA sequences could be distinguished with different PCR primers. DNA extracted from buffy coat was used.
Science 309:1476 (2 Sep. 2005) News Focus “An Earlier Look at Baby's Genes” describes attempts to develop tests for Down Syndrome using maternal blood. Early attempts to detect Down Syndrome using fetal cells from maternal blood were called “just modestly encouraging.” The report also describes work by Dennis Lo to detect the Rh gene in a fetus where it is absent in the mother. Other mutations passed on from the father have reportedly been detected as well, such as cystic fibrosis, beta-thalassemia, a type of dwarfism and Huntington's disease. However, these results have not always been reproducible.
United States Patent Application 20040137470 to Dhallan, Ravinder S, published Jul. 15, 2004, entitled “Methods for detection of genetic disorders,” describes a method for detecting genetic disorders using PCR of known template DNA and restriction analysis. Also described is an enrichment procedure for fetal DNA. It also describes a method used to detect mutations, and chromosomal abnormalities including but not limited to translocation, transversion, monosomy, trisomy, and other aneuploidies, deletion, addition, amplification, fragment, translocation, and rearrangement. Numerous abnormalities can be detected simultaneously. The method is said to provide a non-invasive method to determine the sequence of fetal DNA from a tissue, such as blood, drawn from a pregnant female, and a method for isolating free nucleic acid from a sample containing nucleic acid.